The system stores a mapping of the Spira requirement ID and the Jira issue ID (same for incidents) in the TST_DATA_SYNC_SYSTEM_ARTIFACT_MAPPING table.

When you delete a requirement in Spira, it gets kept in that table to prevent it 'coming back to life' which could be annoying. Just delete the entry in that table that corresponds to the requirement (requirements have ARTIFACT_TYPE_ID = 1 vs. incidents have a value = 3) and it will get recreated.